Browsing by Author "Jikazana, Aphiwe"

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    Data: Hydrodynamics (Reynolds number) determine scaling, nucleation and crystal growth kinetics in membrane distillation crystallisation
    (Cranfield University, 2023-09-05 16:18) McAdam, Ewan; Jikazana, Aphiwe; Campo Moreno, Pablo
    Dataset for journal article "Hydrodynamics (Reynolds number) determine scaling, nucleation and crystal growth kinetics in membrane distillation crystallisation"
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    Hydrodynamics (Reynolds number) determine scaling, nucleation and crystal growth kinetics in membrane distillation crystallisation
    (Elsevier, 2023-07-08) Jikazana, Aphiwe; Campo Moreno, Pablo; McAdam, Ewan
    Reynolds number (Re) has been previously related to several scaling mitigation and crystallisation strategies that offer distinct hypotheses for how Re may regulate the kinetics of nucleation and crystal growth in membrane crystallisation. Such ambiguity has arisen from the present inability to discretely characterise induction time in membrane systems. This study therefore introduces techniques for the detection of induction time, with measurements used to develop a modified power-law relation between nucleation rate and supersaturation to establish how Re can be used to adjust nucleation kinetics. Increasing Re enhanced mass and heat transfer processes which raised permeate flux. The interfacial supersaturation set by the increase in flux, also modified the supersaturation rate at induction for crystals formed in the bulk solution, providing the first direct evidence that it is the supersaturation level set within the boundary layer which controls primary nucleation in the bulk solution. Bulk nucleation rate can therefore be adjusted in proportion to Re. While the extent of scaling was also determined by the interfacial supersaturation set by Re, its formation was shown to be more dependent on the interfacial diffusion coefficient which regulates solute backtransport and the activation energy for nucleation. Through this work we suggest that the nucleation mechanisms underlying scale formation and bulk crystallisation are distinct. The regulation of nucleation rate in the bulk solution by Re is described analytically through classical nucleation theory, while scaling can be mitigated through operation below a critical threshold supersaturation value that determines the rate and type of scaling that prevails. These seemingly distinct strategies can be combined through modifications to T and dT with Re to suppress scaling and offer refined control over the kinetics of nucleation and crystal growth.
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    Supersaturation control in membrane distillation crystallisation
    (Cranfield University, 2022-11) Jikazana, Aphiwe; McAdam, Ewan; Campo Moreno, Pablo
    Membrane distillation crystallisation (MDC) has emerged as a potential alternative to conventional industrial crystallisers. MDC provides controlled hydrodynamics and uniform supersaturation conditions for crystallisation, thereby enhancing scalability, which is desperately lacking in conventional crystallisation systems. Unfortunately, crystallisation near the membrane surface is also associated with inorganic fouling (scaling), which can ultimately lead to process failure. As such, the viability of the technology is dependent on scale-free bulk crystallisation. To date however, scaling mitigation strategies have been based on empirical observations with contradictory postulations regarding the governing crystallisation mechanism(s). In this work, the distinct mechanisms of scaling and bulk crystallisation have been elucidated for the first time. The application of novel inline and online experimental techniques facilitated the development of a mechanistic framework which is able to predict the likelihood of scaling in addition to mediating bulk nucleation kinetics. As such, scale-free operation was achieved at temperatures and hydrodynamic conditions which were previously associated with scaling. This study has therefore broadened the perceived range of kinetic trajectories achievable with MDC and evidenced its applicability to multi- component systems, polymorph selection, and a variety of product specifications. Furthermore, the use of hydrodynamics to decouple nucleation and growth kinetics revealed the potential of MDC to minimise the usual trade-off between product quality and yield in crystallisation systems. While existing scaling mitigation strategies are largely hydrodynamic and thermodynamic in nature, this study has shown that the contribution of crystallisation kinetics (supersaturation rate) to scaling propensity cannot underestimated. Hence, application of the kinetic framework developed could provide more targeted strategies for scaling prevention in various applications such as heat exchangers and reverse osmosis/nanofiltration (RO/NF), where polarisation phenomena are prevalent